Bonded alumina coating for stainless steel

ABSTRACT

A method for manufacturing an alumina-based layer structure having transition regions between layers is disclosed. The method may include ion milling a stainless steel structure surface to partially reduce a metal oxide layer from, and create an exposed portion of, the surface. The method may include oxidizing the exposed portion of the surface to form a crystallized metal oxide bonding layer, growing a crystallized alumina layer onto the metal oxide bonding layer, and diffusing metal from the surface into the crystallized alumina layer, to form a graded aluminate spinel layer. The method may include forming a first transition region from the graded aluminate spinel layer to a crystalline alumina layer, growing the crystalline alumina layer from the first transition region, forming a second transition region from the crystalline alumina layer to an amorphous alumina layer, and growing the amorphous alumina layer from the second transition region.

BACKGROUND

The present disclosure generally relates to coatings of stainless steelobjects. In particular, this disclosure relates to designedalumina-based coatings formed on surfaces of medical-grade stainlesssteel objects.

Stainless steel (also known as inox or “inoxydable” steel) is a steelalloy which may have a minimum of 10.5% chromium content by mass.Stainless steel may not readily corrode, rust or stain with exposure towater or other fluids, and may incorporate metallic elements (other thaniron (Fe)) including, but not limited to, chromium (Cr), nickel (Ni),and molybdenum (Mo), which may be useful in increasing the steel'sresistance to corrosion. Various grades and surface finishes ofstainless steel may be used in particular environments that an alloy mayendure. Stainless steel may be used in applications where both themechanical/structural properties of steel and resistance to corrosionare useful.

“Medical grade” or “surgical” stainless steel may be informal termswhich may refer to certain grades of stainless steel that may be used inbiomedical applications. Common types of stainless steels referred to as“medical grade” may include austenitic 316, 316L and 316LVM. 316, 316Land 316LVM steels may be chromium, nickel and molybdenum alloys of steelthat exhibit relatively high strength and corrosion resistance and maybe a common material choice for biomedical implants and equipment thatare put under pressure. For example, bone fixation screws, prosthesesand body piercing jewelry may be formed from austenitic 316L and 316LVMsteel. 316 steel may also be used in the manufacture and handling offood and pharmaceutical products, where it may be often required tominimize metallic contamination.

Aluminum oxide, commonly known as “alumina”, is a chemical compound ofaluminum and oxygen with the chemical formula AL₂O₃. It is the mostcommonly occurring of several aluminum oxides, and specificallyidentified as aluminum (III) oxide. Alumina may be a ceramic,crystalline material having a high hardness and a high melting point.Alumina may possess properties which may be useful in biomedicalapplications and dental implants, such as being bio-inert (having lowchemical reactivity with bodily tissues and fluids) and mechanicalproperties such as relatively high stability, hardness, and resistanceto wear.

SUMMARY

Various aspects of the present disclosure may be useful for formingdesigned, high adhesion strength alumina-based surfaces on stainlesssteel objects used for medical applications. A stainless steel object,such as a surgical implant or medical instrument manufactured accordingto embodiments of the present disclosure may have a high-hardnesscoating that has limited chemical and biological reactivity with bodilyfluids and/or tissues. A surgical implant or medical instrumentmanufactured according to embodiments may be useful in limitingbiological reactions to stainless steel oxides while exhibitinghigh-strength characteristics of stainless steel.

Embodiments may be directed towards a method for manufacturing, in adeposition chamber, an alumina-based layer structure having transitionregions between layers. The method may include milling, in-situ, astainless steel structure with an ion beam controlled by a set ofparameters that cause the ion beam to remove carbon-based contaminantsand at least partially reduce a metal oxide layer from a surface of thestainless steel structure, to create an exposed portion of the stainlesssteel structure. The method may also include crystallizing, byoxidizing, the exposed portion of the stainless steel structure to forma crystallized metal oxide bonding layer. The method may also includeforming, by growing a crystallized alumina layer onto the metal oxidebonding layer and diffusing metal from the stainless steel surface intothe crystallized alumina layer, a graded aluminate spinel layer on thecrystallized metal oxide bonding layer. The method may also includeforming a first transition region from the graded aluminate spinel layerto a crystalline alumina layer, growing the crystalline alumina layerfrom the first transition region, forming a second transition regionfrom the crystalline alumina layer to an amorphous alumina layer, andgrowing the amorphous alumina layer from the second transition region.

Embodiments may also be directed towards an apparatus. The apparatus mayinclude a stainless steel structure and an at least partiallypolycrystalline alumina-based layer structure bonded to an exposedportion of the stainless steel structure. The layer structure mayinclude a crystallized metal oxide bonding layer formed on the exposedportion of the stainless steel structure, and a graded aluminate spinellayer formed on the crystallized metal oxide bonding layer. The gradedaluminate spinel layer may have metal diffused from the stainless steelsurface, and an at least partially polycrystalline structure. The layerstructure may also have a first transition region extending from thegraded aluminate spinel layer to a crystalline alumina layer, thecrystalline alumina layer formed from the first transition region, asecond transition region extending from the crystalline alumina layer toan amorphous alumina layer, and the amorphous alumina layer formed fromthe second transition region.

Aspects of the various embodiments may be used providing high-strength(atomic) bonding between a stainless steel surface and an amorphousalumina coating due to a continuously graded structure between thestainless steel and the amorphous alumina coating. Aspects of thevarious embodiments may also be useful for providing a hard,wear-resistant, bio-entered outer coating for use on medical implants,such as prosthetic joints, by using existing and proven materialprocessing practices, machining and fabrication technologies.

The above summary is not intended to describe each illustratedembodiment or every implementation of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings included in the present application are incorporated into,and form part of, the specification. They illustrate embodiments of thepresent disclosure and, along with the description, serve to explain theprinciples of the disclosure. The drawings are only illustrative ofcertain embodiments and do not limit the disclosure.

FIG. 1 is an isometric drawing of an alumina-based layer structurebonded to a surface of a stainless steel substrate, according toembodiments of the present disclosure.

FIG. 2 includes six cross-sectional views depicting the results ofprocesses for bonding an alumina-based layer structure to a surface of astainless steel substrate, according to embodiments.

FIG. 3 is a flow diagram illustrating bonding an alumina-based layerstructure to a surface of stainless steel substrate, according toembodiments.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

In the drawings and the Detailed Description, like numbers generallyrefer to like components, parts, steps, and processes

DETAILED DESCRIPTION

Certain embodiments of the present disclosure can be appreciated in thecontext of providing a high adhesion-strength, high hardness, bio-inertcoating for medical devices such as surgical implants, and for stainlesssteel items such as rings and body-piercing jewelry, which may be incontact with bodily fluids and/or tissues. Surgical implants accordingto embodiments may be used to provide, for example, increased bodilystructural support and joint articulation, while controlling oreliminating adverse biological (chemical) reactions to the implant.Surgical implants according to embodiments may include but are notlimited to replacement (prosthetic) joints such as hip, knee andshoulder joints, and reinforcing structures such as bars, pins, nails,screws and buttress plates. While not necessarily limited thereto,embodiments discussed in this context can facilitate an understanding ofvarious aspects of the disclosure.

Certain embodiments may also be directed towards other equipment andassociated applications, such as medical instruments. Medicalinstruments manufactured according to embodiments may provide little (orno) adverse biological (chemical) reactions to the contact of suchinstruments with bodily fluids or tissues during medical procedures.Medical instruments according to embodiments may include but are notlimited to forceps, scalpels, scissors, tweezers, needle holders andlaboratory probes.

Embodiments may also be directed towards non-medical applications suchas equipment used in conjunction with nuclear reactors, marineenvironments, and food or chemical processing applications, to provide ahigh adhesion strength, chemically inert coating. Such coating may beresistive to chemically corrosive environments, and may be used tocontrol (limit) contamination that may occur through the dispersion ofmetallic oxides from stainless steel equipment, and to protect astainless steel surface from corrosion and pitting.

For ease of discussion, the term surgical implant is used herein,however, it is understood that various embodiments may also be usefulwith regards to other medical applications such as medical instrumentsand jewelry items such as rings and body piercing hardware.

Various embodiments of the present disclosure relate to an alumina-basedcoating formed on the surface of a stainless steel object, which mayprovide a high-strength, reliable, bio-inert layer having high hardness,on an outer surface of the object. The stainless steel object may betherefore be useful for providing mechanical stability and support inbiomedical applications such as replacement joints, and various types ofsurgical inserts used for mechanical and structural support.Structurally stable and long-term reliable, biologically non-reactiveperformance of a surgical implant may result from the use of a stainlesssteel object having an alumina-based coating.

A graded alumina-based coating, having high bond strength to a stainlesssteel object, such as a surgical implant, may be useful in preventingdelamination, peeling and cracking of the alumina-based coating while itis exposed to bodily tissues and fluids. Structural integrity of thealumina-based coating may help reduce adverse bodily reactions tometallic oxides (of the stainless steel object) and particles shed fromthe alumina-based coating.

A surgical implant designed according to certain embodiments may becompatible with existing and proven surgical implants, and may be auseful and cost-effective way to protect a patient outfitted with theimplant from harmful bodily reactions, and may prevent or eliminate thereplacement of a defective surgical implant.

Medical/surgical grade stainless steel (austenitic 316, 316L and 316LVM,for example) has been used for biomedical applications because of itsworkability, high tensile strength, ductility, hardness, economicadvantages, general resistance to wear and corrosion and its generalbiocompatibility. Stainless steel may be readily formed and machinedinto a desired shape for a particular medical application. Biomedicalapplications of stainless steel may include, but are not limited to,load-bearing or structural (reconstructive) implants, artificial joints,and medical instruments. Medical/surgical grade stainless steel may alsobe used for aesthetic applications such as rings and body-piercingjewelry.

A variety of physiological reaction types have been reported as a resultof (often prolonged) contact between stainless steel objects and bodilyfluids/tissues. Reaction types may include toxic, hypersensitive,allergenic, autoimmune and carcinogenic reactions, which may result intissue inflammation, organ poisoning or injury, and/or bodily rejectionof an implant device. Metallic ions and oxides, for example nickel ornickel oxide, originating from corrosion (oxidizing) of the stainlesssteel, due to exposure to liquids and electrolytes, may cause at leastsome of the reported adverse effects.

When stainless steel implants are used for extended periods of time orhave bearing or sliding surfaces, e.g., artificial joint implants, wherean abrasive wear processes may be particularly intense, particles ordebris (metallic oxides and ions) may be shed from the implants. As thisdebris is disposed to surrounding tissue, it may be encapsulated,promoting inflammatory reactions and degradation of the tissue, whichmay eventually lead to pain and early loosening of the implant. Metallicoxides and ions may also start to dissolve and diffuse into thebloodstream, which may lead to additional reaction symptoms.

Ceramic materials may be used as a structural material for surgicalimplants, due to their hardness and strength properties, and may be morewear resistant than metallic implants. Ceramic implants may be able towithstand heavy work loads, and may not corrode in response to contactwith bodily fluids and tissues. Ceramic materials may be stiffer andmore brittle than metallic materials, and may therefore, when used asstructural bulk material in implants, be more prone to detrimentalbreaks and fractures than a similar implant formed from metallicmaterials.

A structurally robust, surgical implant may be constructed by using astainless steel alloy as a base or substrate material for the implant,and modifying its surface. One approach may include modifying thesubstrate surface through processes such as ion implantation, gasnitriding and high temperature oxidation. However, this approach mayhave limitations, such as yielding insufficient surface hardness forlong-term wear applications, being prohibitively expensive, and notbeing feasible for use with a variety of desirable substrate materials.

Another approach for designing a material for medical implants may be toapply a ceramic coating to the surface of a metal or metal alloysurface, either to the whole implant or restricted to the surface areasthat are most exposed to abrasive wear. An alumina (AL₂O₃) coating mayprovide a fatigue-resistant surface to a metal substrate which isresilient but sensitive to abrasion. An alumina-based coating may beused to protect a patient's body from direct contact with metallicmedical implants, instruments and jewelry items. The bio-inert aluminacoating may provide a biologically non-reactive barrier between bodilyfluids and/or tissues and stainless steel, and may prevent oxides fromthe stainless steel from being formed and entering bodily tissues andfluids.

A layer structure having an outer layer of aluminum oxide (AL₂O₃) and anintermediate layer comprising titanium nitride (TiN), titaniumcarbonitride (TiCN) or titanium oxycarbonitride (TiCNO) may be formed onthe surface of the various alloys, however this type of layer structuremay start to wear down and peel after prolonged use in artificial jointsor other types of medical implants. Peeling or delaminating may resultin shed debris and exposing only tissues to the substances of thesubstrate, and may also lead to a dramatic increase the rate of the wearprocess, should be degree become trapped between sliding/matingsurfaces. The tendency to peel or delaminate may depend on themechanical properties of both the substrate and the coating, and on howthe coating is bonded to the underlying substrate.

Certain embodiments relate to the formation of an alumina-based coatinghaving a high bond strength to a medical-grade stainless steel surface,as a result of a continuously graded crystalline interface between thestainless steel surface and the alumina coating.

FIG. 1 is an isometric drawing of an alumina-based layer structure 124,having an amorphous alumina layer 118, bonded to a surface of astainless steel substrate 108, according to embodiments of the presentdisclosure. Alumina-based layer structure 124 may be generally useful asa high-strength, bio-inert coating for surgical implants, medicalinstruments, jewelry and body piercing hardware, according toembodiments of the present disclosure.

The process used in forming the alumina-based layer structure 124 mayyield a continuously graded, at least partially polycrystallinestructure, having transition regions between layers, which may result inlayer structure 124 having high hardness, durability and resistance toabrasive wear, cracking, peeling, and delamination from the stainlesssteel substrate 108. The bio-inert properties of the amorphous aluminalayer 118 may make it suitable to shield bodily tissues and fluids frommetallic ions, oxides, and particles shed from the stainless steelsubstrate 108, according to embodiments. The shielding effect of thealumina-based layer structure 124 may be useful in minimizing orreducing adverse bodily reactions to devices such as surgical implantsor medical instruments, and may help to reduce the number of implantsthat may need to be replaced, as a result of adverse reactions.

The stainless steel substrate 108 may include “medical” or “surgical”stainless steels including, but not limited to grades such as AISI 316,316L, 316LVM, which may be chosen for biomedical applications, accordingto embodiments. The crystallized metal oxide bonding layer 112, formedon stainless steel substrate 108, may contain crystalline structuresthat may be useful as seeds for subsequent alumina crystallization. Thegraded aluminate spinel layer 114 may share crystalline structures withmetal oxide bonding layer 112, and may be useful as an intermediatelayer between metal oxide bonding layer 112 and a subsequent aluminalayer. The first transition region 120 may be a graded transitionbetween the graded aluminate spinel layer 114 and adjacent layers, andmay contribute to the overall structural integrity of the alumina-basedlayer structure 124. The crystalline alumina layer 116 may sharecrystalline structures with the first transition region 120, and may beuseful as an intermediate layer between the graded aluminate spinellayer 114 and a subsequent amorphous alumina layer.

The second transition region 122 may be a graded transition between thegraded crystalline alumina layer 116 and a subsequent amorphous aluminalayer, and may contribute to the overall structural integrity of thealumina-based layer structure 124. The amorphous alumina layer 118 mayhave bio-inert properties which may make it suitable to shield bodilytissues and fluids from metallic ions, oxides, and particles from thestainless steel substrate 108, according to embodiments.

FIG. 2 includes six cross-sectional views (201 through 206), consistentwith FIG. 1, depicting the expected results of a set of processes forbonding an alumina-based layer structure 124 (FIG. 1) to a surface of astainless steel substrate 108 (FIG. 1), according to embodiments. Theseviews illustrate an example process; other views and processes may bepossible. An alumina-based layer structure formed by these processes maybe consistent with layer structure 124 (FIG. 1) and may have ahigh-strength (atomic) bond to a stainless steel surface, consistentwith a bond between layer structure 124 (FIG. 1) and stainless steelsubstrate 108.

The progression depicted in cross-sectional views 201 through 206 beginswith a stainless steel substrate 108, in an initial state, having ametal oxide layer 210 (view 201), which may also include carbon-basedcontaminants such as naturally occurring organic matter. Cross-sectionalviews 202 through 206 each depict the results of one or more operationson the stainless steel substrate 108 and/or alumina-based layerstructure (124, FIG. 1). The progression ends with a completedalumina-based layer structure 124 (view 206) bonded to the stainlesssteel substrate 108 (view 206).

Process operations associated with views 201 through 206 may include,but are not limited to ion milling, crystalline layer growth, layeroxidation, reactive sputter deposition of alumina, and metallicdiffusion. The results of one or more process operations may be depictedin each view. For example, a view may depict the formation of one ormore alumina-based layers, which may include process operations ofreactive sputter deposition, metallic diffusion, and crystalline layergrowth. The process operations described herein may be carried out in adeposition chamber, under a partial vacuum condition.

Views 201 through 206 depict the formation of individual layers, such asa crystallized metal oxide bonding layer and a graded aluminate spinellayer, as well as the transition (gradient) regions between individuallayers. The transition (gradient) regions between individual layers mayindicate a structural merging of individual layers, which may result inhigh-strength bonding of the individual layers to adjacent layers, andof the alumina-based layer structure 124 (FIG. 1) to the stainless steelsubstrate 108.

Completed structures may be generally shown in the views as havingrectangular cross-sectional profiles, with surfaces parallel to eachother. This depiction, however, is not limiting; layers may be of anysuitable shape, size and profile, in accordance with specific designcriteria, manufacturing process limitations and tolerances for a givenapplication. The results of the process operations illustrated may notnecessarily be drawn to scale, or be proportional to actual processedlayer structure dimensions. For example, interfaces between layersdepicted as level and regular, may be sloped or irregular, and relativedimensional ratios may vary from those depicted in the figures. Layerthicknesses shown may be drawn for ease of illustration, and may notnecessarily have dimensions proportional to actual processed dimensions.

View 201 depicts a stainless steel substrate 108 having a layer 210which may include amorphous native metal oxides and carbon-basedcontaminants. Carbon-based contaminants may include various forms ofnaturally occurring organic carbon compounds. The amorphous native metaloxides may include oxides of various metals comprising the stainlesssteel substrate 108, including but not limited to nickel oxide,magnesium oxide and chromium oxide. Thickness T1 depicts an initialthickness of layer 210 before any ion milling operation.

Ion milling of a stainless steel structure, which may include stainlesssteel substrate 108 and layer 210, may remove the carbon-basedcontaminants, and reduce amorphous native metal oxides found on thesurface of the stainless steel substrate 108, and may be useful to cleanand expose a portion of the stainless steel structure (stainless steelsubstrate 108 and layer 210) that can support structurally stablecrystalline oxide and alumina growth.

Ion milling of a surface of the stainless steel structure may beperformed using an inert gas, for example, argon 209, which may beinjected into the deposition chamber, and accelerated towards thestainless steel structure surface. A set of ion milling parameters maybe used to specify a particular ion beam incidence angle normal to thestainless steel structure surface, to specify an ion beam energy, inelectron-volts (eV), and to specify an ion milling time interval. Forexample, an ion beam incidence angle may be between 45° and 70° to alocal normal of the stainless steel structure surface, an ion beamenergy may be between 250 (eV) and 500 eV, and the ion beam millingduration may be approximately 10 minutes. TRIM (TRansport of Ions inMatter) simulations may be used to simulate and determine a set of ionmilling parameters suitable for removal of particular types andthicknesses of amorphous native metal oxides and carbon-basedcontaminants.

Ion milling the amorphous native metal oxides may cause them to bechemically reduced, i.e., reduce the number of oxygen atoms within theoxide. For example, Ni₂O₃ may be reduced to NiO. Other types of oxidessuch as molybdenum oxide and chromium oxide, may be similarly reduced.Metal atoms 211 are depicted similarly throughout views 201-206.

View 202 depicts the results of an in-situ ion milling operation whichmay use an inert gas such as argon (209, view 201) for the at leastpartial reduction of amorphous native metal oxides and removal ofcarbon-based contaminants from a surface of the stainless steelstructure (including stainless steel substrate 108 and layer 210).Removal of metal oxides and carbon-based contaminants may create anexposed portion of the stainless steel structure. In certainembodiments, an exposed portion of the stainless steel structure mayhave a remaining metal oxide layer 210 with a thickness T2 that is lessthan the initial thickness T1 (view 201), corresponding to a partialremoval of amorphous native metal oxides. In some embodiments, anexposed portion of the stainless steel structure may have bothcarbon-based contaminants and amorphous native metal oxides completelyremoved. Oxygen plasma (213) may be injected into the depositionchamber, and used, in conjunction with argon, in the ion milling thestainless steel structure, which may continue, with the results shown inview 203.

View 203 depicts the results of crystallizing, by oxidizing with oxygenplasma (213, view 202) injected into the deposition chamber, the exposedportion 210 (view 202) of the stainless steel structure (stainless steelsubstrate 108 and layer 210) to form a crystallized metal oxide bondinglayer 112. Oxygen plasma (213, view 202) may react with the exposedportion of the stainless steel structure, which may include remainingamorphous native metal oxides 210 (view 202) and/or a surface of thestainless steel substrate 108, to form a crystallized metal oxidebonding layer 112.

The formation of crystalline structures in the crystallized metal oxidebonding layer 112 may be promoted by heat from an exothermic oxidationreaction between the oxygen plasma 213 (view 202) and amorphous nativemetal oxides 210 (view 202). Crystalline structures formed in thecrystallized metal oxide bonding layer 112 may be useful as seeds (ortemplate sites) for subsequent alumina crystallization. During theexothermic oxidation reaction, metal atoms 211 may diffuse fromstainless steel substrate 108 into crystallized metal oxide bondinglayer 112. Reactively sputtered alumina 215 is depicted in view 203.

View 204 depicts the results of reactive sputter deposition of alumina(aluminum oxide, AL₂O₃) onto the crystallized metal oxide bonding layer112 in the presence of oxygen plasma 213 to form a graded aluminatespinel layer 114 and a first transition region 120. The graded aluminatespinel layer 114 may share crystalline structures with metal oxidebonding layer 112, and may be useful as an intermediate layer betweenmetal oxide bonding layer 112 and a subsequent alumina layer. Gradedtransitions, including the first transition region 120, between thegraded aluminate spinel layer 114 and adjacent layers may contribute tothe overall structural integrity of the alumina-based layer structure124.

During reactive alumina sputtering, energetic aluminum ions, in thepresence of oxygen plasma, may initiate thermite-like (exothermic)reactions with metal oxides in the crystallized metal oxide bondinglayer 112. These exothermic reactions may generate significant localheat, which may promote the crystallization of reactively sputtered(deposited) alumina and atomic bonding of the crystallized alumina withthe crystallized metal oxide bonding layer 112. The heat from theexothermic reactions may also promote local metal (Cr, Fe, Ni, Mo)diffusion from the stainless steel substrate 108 and the crystallizedmetal oxide bonding layer 112 into the growing graded aluminate spinellayer 114.

As the graded aluminate spinel layer 114 grows, the number of localmetal oxide molecules available to exothermically react with thesputtered alumina may decrease, and eventually local heat may no longerbe released. As local heat is no longer released, a first transitionregion 120 is formed in a top portion of the graded aluminate spinellayer 114, and may extend from the graded aluminate spinel layer 114into a subsequent crystalline alumina layer.

In certain embodiments, the graded aluminate spinel layer 114 may have athickness between 5 and 20 nanometers, and may include crystallizedalumina with an at least partially polycrystalline structure. The gradedaluminate spinel layer 114 may also include metal diffused from thestainless steel substrate 108 surface and from the crystallized metaloxide bonding layer 112. A metal concentration gradient may existbetween a bottom portion and a top portion of the graded aluminatespinel layer 114, with the metal concentration higher near the bottomportion than at the top portion. The first transition region 120 mayencompass a metal concentration gradient between the graded aluminatespinel layer 114 and subsequent crystalline alumina layer.

The graded aluminate spinel layer 114 and first transition region 120may be useful for forming a structurally integrated transition layerbetween the crystallized metal oxide bonding layer 112 and a subsequentcrystalline alumina layer, and may contribute to the overall structuralintegrity of the alumina-based layer structure 124 (FIG. 1).

View 205 depicts the results of reactive sputter deposition of aluminaonto the first transition region 120 in the presence of oxygen plasma213 (view 202) to form a crystalline alumina layer 116 and a secondtransition region 122. The crystalline alumina layer 116 may sharecrystalline structures with the first transition region 120, and may beuseful as an intermediate layer between the graded aluminate spinellayer 114 and a subsequent amorphous alumina layer. Graded transitions,including the second transition region 122, may contribute to theoverall structural integrity of the alumina-based layer structure 124.

During reactive alumina sputtering, latent heat from prior exothermicreactions may continue to promote the crystallization of reactivelysputtered (deposited) alumina and atomic bonding of the crystallizedalumina with the first transition region 120. As the crystalline aluminalayer 116 grows, and cools, in response to a cessation of exothermicreactions, a second transition region 122 may be formed in a top portionof the crystalline alumina layer 116, and may extend from thecrystalline alumina layer 116 into a subsequent amorphous alumina layer.

The crystalline alumina layer 116 may include crystallized alumina withan at least partially polycrystalline structure. The second transitionregion 122 may encompass a crystalline structure gradient between thecrystalline alumina layer 116 and subsequent amorphous alumina layer,with the crystalline structure proportion higher near a bottom portionthan at a top portion of the second transition region 122. Thecrystallized alumina layer may include at least one cubic allotrope ofaluminum oxide.

The crystalline alumina layer 116 and second transition region 122 maybe useful for forming a structurally integrated transition layer betweenthe graded aluminate spinel layer 114 and a subsequent amorphous aluminalayer, and may contribute to the overall structural integrity of thealumina-based layer structure 124 (FIG. 1).

View 206 depicts the results of reactive sputter deposition of aluminaonto the second transition region 122 through in the presence of oxygenplasma 213 (view 202) to form an amorphous alumina layer 118. Theamorphous alumina layer 118 may share an amorphous structure with secondtransition region 122, and may be useful as a biologically andchemically inert surface layer of an alumina-based layer structure 124(FIG. 1). The biological and chemical inertness of amorphous aluminalayer 118 may make it useful as a coating for stainless steel objectsand equipment used in medical applications, food, drug, pharmaceuticaland chemical processing, and marine environments

Amorphous alumina layer 118 may have a high hardness, resistance towear, and be resistant to cracking, peeling, or delaminating fromstainless steel substrate 108, due to the overall structural integrityof the graded layer structure 124 used to attach it to stainless steelsubstrate 108.

FIG. 3 is a flow diagram consistent with FIG. 1, 2, illustratingoperations for bonding an alumina-based layer structure to a surface ofstainless steel substrate (108, FIG. 1), according to embodiments.

The process 300 moves from start 302 to operation 304. Operation 304generally refers to the process operations that involve ion milling astainless steel structure, which may correspond to the views provided by201, 202 (FIG. 2) and their associated descriptions. The ion millingoperation may be controlled through a set of parameters that define theenergy, angle of incidence, and exposure time of the ion beam during theion milling process. Ion milling may be useful in cleaning and preparinga surface of a stainless steel object for growth and deposition ofsubsequent material layers. Once the stainless steel structure has beenion milled, the process moves to operation 306.

Operation 306 generally refers to the process operations that involvethe formation and crystallization of a metal oxide bonding layer, whichmay correspond to the view provided by 203 (FIG. 2) and its associateddescription. The metal oxide bonding layer may formed from existingamorphous native metal oxides and/or an exposed surface of the stainlesssteel structure, through the use of oxygen plasma in a depositionchamber. The metal oxide bonding layer may be useful in providing acrystalline layer that is structurally integrated with the stainlesssteel substrate surface, for further growth and formation of subsequentstructural layers. Once the metal oxide bonding layer has been formed incrystallized, the process moves to operation 308.

Operation 308 generally refers to the process operations that involveforming a graded aluminate spinel layer, which may correspond to theview provided by 204 (FIG. 2) and its associated description. The gradedaluminate spinel layer may be formed through the reactive sputterdeposition of alumina, in conjunction with heat provided by anexothermic reaction between sputtered alumina and native metal oxides.Heat from the exothermic reaction may result in crystallization of thesputtered alumina, and may also promote diffusion of metal atoms fromthe stainless steel substrate surface into the graded aluminate spinellayer. After the graded aluminate spinel layer has been formed, theprocess moves to operation 310.

Operation 310 generally refers to the process operations that involveforming a first transition region, which may correspond to the viewsprovided by 204 (FIG. 2) and its associated description. The firsttransition region denotes a metal concentration gradient between and astructurally integrated joining of the graded aluminate spinel layer anda subsequent crystalline alumina layer. After the first transitionregion is formed, the process moves to operation 312.

Operation 312 generally refers to the process operations that involvegrowing a crystalline alumina layer, which may correspond to the viewprovided by 205 (FIG. 2) and its associated description. The crystallinealumina layer may be formed through the reactive sputter deposition ofalumina, in conjunction with latent heat provided by prior exothermicreactions between sputtered alumina and native metal oxides. The latentheat may result in crystallization of the sputtered alumina. After thecrystalline alumina layer is grown, the process moves to operation 314.

Operation 314 generally refers to the process operations that involveforming a second transition region, which may correspond to the viewprovided by 205 (FIG. 2) and its associated description. The secondtransition region denotes a crystalline structure gradient between and astructurally integrated joining of the crystalline alumina layer and asubsequent amorphous alumina layer. After the second transition regionis formed, the process moves to operation 316.

Operation 316 generally refers to the process operations that involvegrowing an amorphous alumina layer, which may correspond to the viewprovided by 206 (FIG. 2) and its associated description. The amorphousalumina layer may be formed through the reactive sputter deposition ofalumina onto the second transition region. The amorphous alumina layermay share an amorphous structure with the second transition region, andmay be useful as a biologically and chemically inert layer on astainless steel substrate. The amorphous alumina layer may also possessproperties of high hardness, and resistance to cracking, chipping anddelamination from the stainless steel substrate, which may make ituseful as a coating in a variety of medical, chemical and foodprocessing applications. After the amorphous alumina layer is grown, theprocess 300 may end at block 318.

The descriptions of the various embodiments of the present disclosurehave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. An apparatus comprising: a stainless steelstructure; an at least partially polycrystalline alumina-based layerstructure bonded to an exposed portion of the stainless steel structure,the layer structure including: a crystallized metal oxide bonding layerformed on the exposed portion of the stainless steel structure; a gradedaluminate spinel layer formed on the crystallized metal oxide bondinglayer, the graded aluminate spinel layer having: metal diffused from thestainless steel surface; and an at least partially polycrystallinestructure; a crystalline alumina layer; a first transition regionincludes a metal concentration gradient extending from the gradedaluminate spinel layer to the crystalline alumina layer; an amorphousalumina layer; and a second transition region includes a crystallinestructure gradient extending from the crystalline alumina layer to theamorphous alumina layer.
 2. The apparatus of claim 1, wherein theapparatus is a medical device and the stainless steel structurecomprises medical-grade stainless steel.
 3. The apparatus of claim 1,wherein the amorphous alumina layer is biologically inert.
 4. Theapparatus of claim 1, wherein the alumina-based layer structure isatomically bonded to the stainless steel structure.